This invention relates to a vapor compression refrigeration cycle and, in particular, to an expansion device for throttling refrigerant vapors moving between a pair of heat exchangers which permit the function of the exchangers to be automatically reversed when the cycle operation is changed from a cooling mode to a heating mode.
Normally, in a conventional cooling cycle, slightly superheated refrigerant vapors are discharged from a compressor into a first heat exchanger (condenser) wherein the refrigerant vapors are reduced to a subcooled liquid at a constant temperature. The heat of condensation is rejected from the system into a sink, such as ambient air or the like, and the liquid refrigerant throttled to a lower temperature and pressure. The low temperature refrigerant is then brought through a second heat exchanger (evaporator) in heat transfer relationship with a higher temperature substance to accomplish the desired cooling thereof. Lastly, the evaporate is drawn from the second exchanger by the suction side of the compressor and the cycle is repeated. It has long been recognized that the energy rejected from the cycle during condensation can be used to provide heating.
Typically, to convert the cooling cycle to a "heat pump," the duty of the two heat exchangers is thermodynamically reversed. To achieve this result, the direction of refrigerant flow through the system is reversed by changing the connection between the suction and discharge side of the compressor and the two exchangers, as for example, by repositioning a four-way valve interconnecting the exchangers with the inlet and outlet to the compressor. The cooling condenser now functions as an evaporator, while the cooling evaporator serves as a heating condenser. To complete the thermodynamic reversal, the refrigerant must be throttled in the opposite direction between exchangers. Reversible refrigerant cycles have heretofore generally utilized either a capillary tube or a double expansion valve and bypass system positioned in the supply line connecting the two heat exchangers to accomplish throttling in either direction.
The capillary tube relies upon a fixed geometry to achieve throttling in either direction. The length of the capillary tubes required in a refrigeration system is excessively long and accommodating a tube of this length within the system poses a problem. Secondly, and more importantly, the flow rate that can be supported by a conventional capillary tube is limited. Once the velocity of the refrigerant reaches sonic velocity at the end of the tube, the flow becomes choked. At this time, the flow attains a maximum velocity and the tube will not respond to further changes in inlet or outlet conditions. As a consequence, the usage of a capillary tube in a reversible refrigeration system imposes serious limitation upon the operational range of the system.
In the double expansion valve arrangement, two opposed expansion valves are positioned within the refrigerant supply line extending between the two heat exchangers. A valve operated bypass is also positioned about each expansion valve, which, when the cycle is reversed, is regulated by a relatively complex control network to alternatively utilize one expansion device and bypass the other. The double bypass system thus requires expensive hardware to implement and a complex control network to operate which, because of its complexity, increases the likelihood of a system failure.